The present invention relates to small size electrochemical cells of high capacity.
In the prior art of wet-type electrochemical cells, for the positive electrode case an, austenitic ferrite stainless steel material (SUS329J1), which has an aluminum layer inside of the case, is used (hereinafter briefly specified as Al-SUS clod materials) as referenced in the Japanese laid-open patent 62-94908.
FIG. 5 shows a sample constitution of an electric double-layer capacitor in a prior art electrochemical cell. The polarization electrode 3, 3' comprises actuated carbon fiber cloth, on which of one side of each is formed a collector 4, 4' made of aluminum through a plasma spraying method, and the actuated carbon fiber cloth is welded to positive electrode case 1, 2 and negative electrode case 7 through, for example, a laser welding method. The electrodes 3, 3' are coupled to each other by way of a separator 5, and assembled by bending an upper side of the positive electrode case towards the inside surface after organic electrolytic liquid 6 is poured into it. As for the electrolytic liquid, a solvent is used in which a tetra-alkylammonium compound, tetra-alkylphosphonium compound etc. is melted into solvents of aprotic r-butyl lactone ethylene carbonate, propylene carbonate, etc.
When the aforesaid prior art electrochemical cell is usually used in the voltage range of 2-2.8 V, anodization of the inside of the positive electrode case, which is made of the element of stainless steel only, is accelerated, solution of metal ion becomes more active, and thus higher impedance in the cell or decrease in electric capacity can be observed. In order to suppress the anodization an, aluminum layer is set inside of the positive electrode case.
Because of the above, a simple substance of JIS standard SUS329J1, or SUS449J1 cannot be used as the materials for the positive electrode case and instead a laminated of two different metals of stainless steel and aluminum is used as the materials for the positive electrode case. However, a uniform and equally thick aluminum layer is difficult to achieve during manufacturing and multiple lamination processes are necessary for its production. Therefore its cost is more than several times as much as of a simple substance stainless steel.
Moreover, during the pressing process, an aluminum layer sometimes is placed over the upper side edge of the positive electrode case. At the assembling process the positive electrode case 1, 2 and the negative electrode case by bending the positive electrode case towards the inside at the edge and by shielding the cell, aluminum at the inside of the positive electrode case comes off and small fragments of aluminum become coupled with the negative electrode case 3. This becomes a reason for a short-circuit. Even when an aluminum layer does not cover the edge of the stainless steel, it invites divergence in shielding between the positive electrode case and the negative electrode case in, and this becomes too a reason for a short-circuit.
According to the present invention, during use of an electrochemical cell with organic electrolytic liquid at a level of 2-2.8 V, anodization of a positive electrode cell without using a aluminum layer inside of it can be suppressed, the product cost can be decreased and therefore its productivity can be improved.